1. Field of the Invention
The present invention relates to a liquid crystal display device, particularly to a reflective liquid crystal display device having a reflection function, and a semi-transmission type liquid crystal display device having a reflection function and transmission function.
2. Description of the Related Art
In recent years, a liquid crystal display device has been applied to various apparatuses such as a personal computer, television, word processor, and mobile phone. An application range of the liquid crystal display device has been broadened, whereas a demand for enhanced functions such as a small size, power saving, and low cost is increasing. To satisfy this demand, development of a reflective liquid crystal display device has been advanced. Since the reflective liquid crystal display device uses an external light to display an image, an internal light source such as a back light unit is not required.
In the reflective liquid crystal display device, the external light is reflected by a reflective plate and optically modulated by a liquid crystal layer so that the image is displayed. A brightness of the external light depends on an installation environment of the liquid crystal display device, and is not stable as in a back light. Therefore, to prevent a light intensity of the external light from being attenuated as much as possible is important for display of a bright image. Particularly, a reflection property of the reflective plate largely influences the attenuation of the light intensity. Therefore, optimization is attempted in order to obtain the reflection property for efficiently reflecting the external light incident at any angle.
As one example of the optimization, it is proposed to dispose an undulation on a reflective surface of the reflective plate as shown in FIG. 10. That is, the undulation of the reflective surface controls scattering of a reflected light so as to concentrate the reflected light in a certain range of area, and raise a reflected light intensity with respect to a specific observation direction.
In actual manufacturing, the reflective surface having the aforementioned undulation is obtained by disposing the reflective plate on a main scattering portion including a plurality of irregularly arranged circular protrusions. The reflection property of the reflective plate is substantially optimum, when a diameter d of the protrusion is set to a range of 3 to 20 μm, and a height H of the protrusion is set to a range of 0.6 to 1.2 μm. However, when the undulation of the reflective surface, that is, a difference of elevation exceeds 1 μm by the aforementioned structure with respect to a small lateral width of about 10 μm, this induces an alignment defect of a liquid crystal, and an image contrast is deteriorated.
As a countermeasure, if the difference of elevation of the reflective surface is limited to about 0.5 μm with respect to the aforementioned lateral width, the deterioration of an image quality can be avoided. However, when such countermeasure is taken, an inclination angle of the reflective surface decreases. Therefore, the reflected light intensity in the vicinity of regular reflection relatively increases. As a result, a range of an angle of a field of view in which a sufficient reflected light intensity is obtained is narrowed. That is, this countermeasure is not practical because the reflection property of the reflective plate is not optimized.